10 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



No detailed analysis is found in the literature relating to differentiation of the 

 mesenchymal cells in the muscles into their sheaths at various embryonic periods. 

 Only a comparison of the typical development of the mesenchyme in different 

 organs with that taking place in specimens under experiment would enable us to 

 realize fully the extent and importance of the changes taking place under experi- 

 mental conditions. To follow, however, step by step, the differences between the 

 normal development of the mesenchyme and its response under experimental con- 

 ditions in the different organs would mean too great an extension of the present 

 paper. However interesting might be the results of such systematic compara- 

 tive study, it would require much time, and the presentation of the general 

 results obtained, controlled and verified during the last three years, would neces- 

 sarily have been further delayed. Moreover, the progress of changes in the mesen- 

 chymal constituents in muscles and other organs in specimens under experiment is 

 so pronounced, and the final results of these changes so striking, that it seemed to 

 me permissible and desirable to give here a synopsis of these changes, and at the 

 same time to analyze as far as possible their significance. I will therefore confine 

 myself to a description of the gradual changes which take place in the mesenchyme 

 of the embryos under experiment and which finally lead to such fundamental trans- 

 formations that in some cases the characteristic structure of definite organs can be 

 hardly recognized. 



An indication of an abnormally intense activity in the mesenchyme may be 

 observed as early as the second day after a successful grafting. Figure 1 represents 

 a small muscular area of the wing at this stage. The grafting having been made 

 at the end of the seventh day of incubation, the embryo is now about 9 days old. 

 The cross-striation of the muscular fibers is distinct. Unlike conditions found in 

 normal embryos, groups of muscle-fibers and individual fibers in the various parts 

 of the embryo body are in many places separated by quite large interstices. They 

 give an impression of being cedematous. In the interstices between the muscular 

 fibers mesenchymal tissue is found which shows an intense proliferation ; numerous 

 mitoses are present. In regions immediately adjacent to the muscular fibers the 

 mesenchyme appears in the form of a more or less loose syncytium, the cells of which 

 have an elongated shape and are closely apposed to the fibers. A sheath of mesen- 

 chymal cellular network is thereby furnished to the muscular fibers, which contain 

 rather numerous nuclei. There is no difficulty in distinguishing the nuclei of the 

 mesenchymal cells from those of the muscular fibers. Their flattened shape and 

 superficial position, together with their characteristic structure, make the identifi- 

 cation of the nuclei of the mesenchymal cells quite easy. 



A different appearance is offered by the mesenchyme situated in the enlarged 

 interstices between the muscle-fibers. There neither cytoplasm nor nuclei are 

 flattened or distended; on the contrary, the mesenchymal cells appear in the form 

 of huge bodies, from which numerous cytoplasmic processes emerge. A syncytium 

 with rather short meshes is thereby formed. Similar areas in the muscles closely 

 resemble the figures in Godlevsky's paper (1902) given as an illustration of the 

 results brought about by physiological degeneration in muscles. If the loose tissue 



